In a vehicle, a remote keyless entry system and a smart entry system have been put in practical use, as a keyless entry system. In the remote keyless entry system, when a user of the vehicle depresses a button of a mobile device, an electric wave including information specific to the vehicle is transmitted from the mobile device, and the electric wave is received by an in-vehicle device to conduct authentication of the mobile device, for example, thereby conducting automatic control of vehicle equipment, such as a lock or unlock of a vehicle door. In the smart entry system, when a user who carries the mobile device as an electronic key enters a radio communication area around the vehicle, a reply signal is transmitted from the mobile device, and the reply signal is received by the in-vehicle device to conduct authentication, for example, thereby permitting unlock of a door. The user is allowed to unlock the door, for example, by merely operating a switch that is disposed outside of the door.
In the above keyless entry system, use of plural frequency channels is proposed in order to ensure certainty of a communication. For example, JP 4-315681A proposes a communication system, in which the user operates a changeover switch that is disposed in the mobile device to change over the frequency channels of the electric wave that is transmitted from the mobile device. However, this method is not practical, because the frequency channel of the transmission electric wave is changed over based on the operation of the changeover switch by the user. The user must always recognize whether the communication has been successfully conducted or not.
Under the circumstances, EP 1362753 A1 proposes a communication system in which, for example, when the in-vehicle device detects the noise level of the frequency channel of the transmission electric wave, and determines that the communication is jammed by the noise, the mobile device changes over the frequency channel of the transmission electric wave. Thus, the communication is automatically conducted through the frequency channel that is small in the noise level, thereby making it highly possible to successfully perform the communication.
However, when the in-vehicle device detects the noise level in addition to the original communication, a ratio of a time during which the in-vehicle device is activated to operate to a unit time becomes large. As a result, the power consumption of the in-vehicle device as well as the overall system is increased. On the other hand, the keyless entry system frequently operates in a state where an engine of the vehicle is not driven, that is, the keyless entry system operates by only a supply voltage from a battery power supply. It is not preferred that, the power consumption is increased.
Also, in the remote keyless entry system, because the user depresses the button of the mobile device to transmit the transmission electric wave from the mobile device, the in-vehicle device cannot determine when the transmission electric wave is transmitted from the mobile device. For this reason, for example, it is assumed that the transmission electric wave is transmitted from the mobile device when the in-vehicle device is in the noise level detecting operation. In this case, not only the in-vehicle device misses an opportunity to receive the transmission electric wave, but also the in-vehicle device may erroneously detects the transmission electric wave level as the noise level as the occasion demands, and changes the frequency channel in error.
That is, when the frequency channel of the communication is changed over when the noise level is detected and it is determined that the communication is jammed by the noise, the communication between the mobile device and the in-vehicle device is not successfully conducted although the power consumption is increased.